The following applications are hereby incorporated by reference into this application as if set forth herein in full: (1) U.S. patent application Ser. No. 09/240,751, entitled xe2x80x9cElectric Utility Network With Superconducting Magnetic Energy Storagexe2x80x9d and filed on Jan. 29, 1999; (2) U.S. Provisional Application No. 60/117,784, entitled xe2x80x9cElectric Utility Network With Superconducting Magnetic Energy Storagexe2x80x9d and filed on Jan. 29, 1999; (3) U.S. patent application Ser. No. 09/449,505, entitled xe2x80x9cMethod and Apparatus for Discharging a Superconducting Magnetxe2x80x9d and filed on Nov. 24, 1999; (4) U.S. patent application Ser. No. 09/449,436, entitled xe2x80x9cMethod and Apparatus for Controlling a Phase Anglexe2x80x9d and filed on Nov. 24, 1999; (5) U.S. patent application Ser. No. 09/449,378, entitled xe2x80x9cCapacitor Switchingxe2x80x9d and filed on Nov. 24, 1999; (6) U.S. patent application Ser. No. 09/449,375, entitled xe2x80x9cMethod and Apparatus for Providing Power to a Utility Networkxe2x80x9d and filed on Nov. 24, 1999; (7) U.S. patent application Ser. No. 09/449,435, entitled xe2x80x9cElectric Utility System with Superconducting Magnetic Energy Storagexe2x80x9d and filed on Nov. 24, 1999; and (8) U.S. Provisional Application No. 60/167,377, entitled xe2x80x9cVoltage Regulation of a Utility Power Networkxe2x80x9d and filed on Nov. 24, 1999.
This invention relates to electric power utility networks including generating systems, transmission systems, and distribution systems serving loads. In particular, the invention relates to controlling the transfer of energy to and from a utility power network. Energy storage devices, including capacitor banks and superconducting magnetic energy storage devices (SMES), are used to provide power to a utility power network in order to compensate for power shortfalls or voltage instability problems on the network. For example, in the event of a fault or outage on the network, power may be transferred from an energy storage device to the network to ensure that the amount of power on the network remains within acceptable limits.
The invention features a system for controlling a power compensation device, such as an inverter connected to a utility power network, to operate in an xe2x80x9coverloadxe2x80x9d mode. Operating in an overload mode means operating the power compensation device in excess of its maximum steady-state power delivery characteristic (e.g., power delivery rating). This reduces the cost of heat dissipation elements in the compensating device and reduces the number of solid state switching devices required therein.
In one aspect, the invention is a system that includes a controller which controls a reactive power compensation device to deliver, for a first period of time and in response to a detected change in a nominal voltage, reactive power to the utility power network. In a second period of time following the first period of time, the controller controls the reactive power compensation device to provide reactive power to the utility power network at a level that is a factor N(N greater than 1) greater than a maximum power capability characteristic of the reactive power compensation device.
In another aspect, the invention is directed to providing power compensation from a power compensation device to a utility power network carrying a nominal voltage, the power compensation device having a steady-state power delivery characteristic. This aspect features detecting a change of a predetermined magnitude in the nominal voltage on the utility power network, and controlling the power compensation device to deliver, for a first period of time and in response to the detected change in the nominal voltage, reactive power to the utility power network. The power compensation device is controlled to deliver, for a second period of time following the first period of time, reactive power to the utility power network at a level that is a factor N(N greater than 1) greater than the steady-state power delivery characteristic of the power compensation device.
Having detected and reacted to a change of a predetermined magnitude in the nominal voltage on the utility power network by increasing injected power to a level that is as much as N times higher than the maximum steady-state power delivery characteristic of the compensation device, power injection of the compensating device can be purposefully and gradually reduced to the maximum steady-state value so as not to include a transient response by the network that could result in voltage instability and/or other undesirable events.
Among other advantages, these aspects of the invention provide an approach for operating a reactive power compensation device in an overload mode for a maximum period of time without incurring an abrupt, step-like change in inverter current at the time the overload capability of the compensating device has been expended, thereby forcing the compensating device""s current to be at or below a specified level. Thus, as noted, the invention reduces the possibility of undesirable transients (e.g., ringing oscillations) in the utility power network. Furthermore, a substantially optimum ramp down profile can be determined on the basis of the characteristic impedance of the network.
Embodiments of the foregoing aspects of the invention may include one or more of the following features. During the first period of time, the compensation device provides real power and reactive power to the utility power network. After the second period of time, the reactive power from the compensation device is non-discontinuously decreased to the steady-state power delivery characteristic. The factor N is generally determined on the basis of a transient thermal capacity characteristic (e.g., a 1% rating) of the compensation device. The second period of time is determined on the basis of the ability of the compensation device to absorb thermal energy. The ramp down profile may be determined on the basis of the characteristic impedance of the network. The characteristic impedance of the network may be determined using known characteristics of the network. Alternatively, the reactive power compensation device can apply a stimulus to the network and a response measured.
These and other features and advantages of the invention will be apparent from the following description, drawings and claims.